1. Field of the Invention
The present invention relates to an X-ray multi-layer mirror used as a reflecting mirror in a projection optical system or the like and an X-ray exposure apparatus using the X-ray multi-layer mirror.
2. Related Background Art
In an X-ray exposure apparatus using a soft X-ray having a wavelength of 13.5 nm or less, a multi-layer film structure is necessarily used to obtain a mirror having a high reflectance outside a total reflection incident angle range. There has been widely known a structure using a distributed Bragg reflecting material similar to a ¼-wavelength stack having an entirely constant film thickness. For example, when a mirror for an X-ray having a wavelength of 13.4 nm includes an Mo/Si alternate layer having a constant film thickness fundamental structure, which is composed of an Mo layer having a film thickness of 2.8 nm and an Si layer having a film thickness of 4.1 nm, a maximum theoretical reflectance exceeds 70% (see Japanese Patent Application Laid-open No. 2001-51106).
However, in the Mo/Si alternate layer having the constant film thickness fundamental structure, a range of an X-ray incident angle at which a high reflectance is obtained is only within about 10 degrees. Therefore, with respect to a light beam incident at an incident angle outside this range, a significant reduction in reflectance is caused.
According to Japanese Patent Application Laid-open No. 2001-51106, there is proposed an alternate multi-layer film having a non-uniform film thickness structure in which Ru or the like is used as a third material for the Mo/Si alternate layer and the thickness of each layer is changed by numeric repetition optimization processing, for example, the film thickness of the Mo layer is changed from about 1 nm or less to near 2 nm.
However, in the Mo/Si alternate layer which is widely used for a mirror for the soft X-ray having the wavelength of 13.5 nm and made of two materials of Mo and Si, Mo and Si are easy to react with each other and form a compound of MoSix at an interface. Therefore, when a design film thickness obtained by optimization processing reduces, a design value is significantly different from a reflection characteristic after film formation in some cases.